# What does “\” mean in math

In a Linear Algebra textbook I am reading, the following is stated: $b\notin \operatorname{span}(A \cup \{a\})\setminus \operatorname{span}(A)$. It does so without explaining what "$\setminus$" means. I apologize if this question does not belong here but I just want to understand what it means. I can close the question if someone just comments on its meaning.

• – Zev Chonoles Feb 9 '14 at 18:06

$\setminus$ (\setminus) as its name implies is the set-theoretic difference: $A\setminus B$ is the set of all elements which are in $A$ but not in $B$. ($A-B$ is also used for this.) Be careful to not confuse $\setminus$ with $/$ (quotient).